Let . Determine the number of positive integers with such that is divisible by . (VMO, 2008)
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This seems incorrect. The answer should be 8 . If you restricted to n ≤ m or nonnegative integers n , then 9 would be correct.
2 0 0 7 2 0 0 8 = 3 4 0 1 6 ⋅ 2 2 3 2 0 0 8 is the prime factorization. But it's clear that only one of the three factors can be divisible by 3 and only one can be divisible by 2 2 3 , so one of the factors n , 2 n + 1 , 5 n + 2 has to be divisible by 3 4 0 1 6 and one of them has to be divisible by 2 2 3 2 0 0 8 . This is necessary and sufficient for the product to be divisible by m .
So this is a Chinese Remainder Theorem computation. There are 9 total solutions mod m , since there are 3 choices for the factor in both conditions, but one of the solutions is 0 , which is ruled out by the problem. So the correct answer is 8 .